See -> Add a payment method to your Apple ID
Add a payment method to your Apple ID – Apple Support (UK)
- if you want to use your Apple ID balance, you need to set your Apple ID balance as your default payment.
Additional:
Try to optimize the range of the mobile network, see this Apple article:
If you see No Service, Searching, or SOS on your iPhone or iPad
If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support
Low Power Mode is On.
Charge the iPhone & wait until the battery is fully charged (100%).
The Low Power Mode will turned off automatically.